Mazhab/Madhab never mentioned in Quran but Deen has been mentioned.
Deen means a complete system, Social- Economic- Religious, whereas Mazhab only religious portion within the fold of Deen.
Quran Chapter 5 Surah Al Maidah verse 3:
Today I (AllahAllah The Holy god of Muhammad, who revealed the Quran to him. In pre-Islamic Arabian culture, he had two daughters. He is al-ilāh. Il, el, or Eloah (Elohim= King in Torah) are the Semitic names of Allah. Allah's tawḥīd, wāḥid and aḥad were revealed to his Rusul.) have perfected your deen for you and completed my Blessing on you and approved Islam as your deen.
More Stories
Muhammad’s order to “kill any Jew that comes under your power”
Muslims for Secular Democracy: Founding Declaration (02/10/2003)
Muslim Civilization and Muslim Constructive Role In the Contemporary World Civilization